Electrophotography, which was invented by C. F. Carlson, has advantages such as instantaneousness and the ability to give high-quality images having high storability. Because of this, electrophotography is extensively used not only in the field of copiers but in the field of various printers and facsimile. Recently, electrophotography has come to be extensively used also in digital multifunction appliances. Applications thereof are spreading widely. With respect to photoreceptors serving as the core of electrophotography, use is being mainly made of photoreceptors employing an organic photoconductive material which has advantages such as non-polluting properties, ease of film formation, and ease of production. Of these, the so-called multilayer type photoreceptor having superposed layers including a charge-generating layer and a charge-transporting layer has the following advantages. It can be obtained as a higher-sensitivity photoreceptor. There is a wide choice of materials, and a highly safe photoreceptor is hence obtained. Furthermore, the coating operations contribute to high productivity and are relatively advantageous in cost. Because of these advantages, photoreceptors of the multilayer type are mainly used at present and are being produced in large quantities.
On the other hand, digitization for image formation is proceeding rapidly in order to obtain images of higher quality or to store input images or edit the images at will. So far, the apparatus which digitally form images have been limited to the laser printers or LED printers as output apparatus for word processors or personal computers and to some kinds of color laser copiers, etc. Recently, however, digitization has been almost completely achieved also in the field of common copiers, in which image formation in an analogue manner was mainly used hitherto.
In the case of conducting such digital image formation, a laser light or an LED light is mainly used as a light source for optical digital-signal input to a photoreceptor. Light sources for optical input which are presently in wide use emit near infrared light having a wavelength of 780 nm or 660 nm or a light having a long wavelength close to these. In recent years, blue lasers have been put to practical use, and a light having a short wavelength of 400-500 nm has become usable as a light source for optical input. Photoreceptors for use in digital image formation are required to have effective sensitivity to such various light sources for optical input, and a wide variety of materials have hitherto been investigated. Besides being high in sensitivity, the photoreceptors are required to have basic properties such as sufficient electrification characteristics, reduced dark decay after charging, low residual potential, and satisfactory stability of these properties during repetitions of use.
Especially in repetitions of use in copiers or printers, there is a problem that the photosensitive layer deteriorates gradually. There is hence a desire for the property of being less damaged by repetitions of use and for stable electrical characteristics. These properties greatly depend on the charge-generating substance, charge-transporting substance, additives, and binder resins. As the charge-generating substance, phthalocyanine pigments and azo pigments are mainly used because of the necessity of having sensitivity to light sources for optical input. As the charge-transporting substance, various kinds of substances are known. Of these, amine compounds are extensively utilized because they have an exceedingly low residual potential (see, for example, patent document 1 and patent document 2). As the additives, various ones are known. Well known of these are ones having the effect of enhancing ozone resistance (see, for example, patent document 3 and patent document 4). Furthermore, with respect to binder resins for use in photosensitive layers, in particular, charge-transporting layers, polycarbonate resins and polyarylate resins are advantageously used (see, for example, patent document 5).
Patent Document 1: JP-A-2000-075517
Patent Document 2: JP-A-2002-040688
Patent Document 3: Japanese Patent No. 2644278
Patent Document 4: JP-A-9-265194
Patent Document 5: JP-A-2000-075517